A typically adopted method when growing a thin film on, for example, one main surface of a semiconductor substrate for forming a semiconductor element is to expose the one main surface of the substrate to source gases that will constitute a thin film to be deposited while heating the substrate. The source gases include, for example, a metalorganic compound of group III nitride semiconductor to be a cation and a group V element to be an anion. By supplying these source gases onto the main surface of the semiconductor substrate having been heated, a thin film is grown on the one main surface of the semiconductor substrate.
The method of growing a thin film as described above is called chemical vapor deposition. The chemical vapor deposition is a kind of epitaxial crystal growth. When growing (depositing) a thin film, a thin film of a material different from that constituting the substrate may be grown by chemical vapor deposition on the substrate, for example. Such a technique for growing by chemical vapor deposition a thin film of a material different from that constituting the substrate is called heteroepitaxial growth. A film deposited (grown) is called a heteroepitaxial film, and an interface between the substrate and the deposited film is called a heterointerface. As will be described later, the substrate as used herein refers to a member on which a film is to be deposited, and includes both a single-layer substrate, on one main surface of which a thin film is to be deposited, and a substrate, on one main surface of which one or more layers of thin films are already deposited, with another thin film to be deposited on one main surface of the thin films (a so-called substrate with an epitaxial film; herein defined as an epi-wafer).
In the case of depositing a heteroepitaxial film as described above, a gallium indium nitride (InGaN) thin film heteroepitaxially grown on gallium nitride (GaN) crystals as shown in, for example, Group III Nitride Semiconductor, Chapter 12 (Non-Patent Document 1) is regarded as crystals that are disadvantageously less likely to achieve a sharp change in composition at the interface between InGaN and GaN. More specifically, an InGaN thin film of an intended composition is not formed on GaN crystals, but when forming an InGaN layer of about 5 nm in thickness, for example, a transition layer with a particularly low indium (In) composition is present and extends for about 1 to 2 nm in a direction across the thickness from the interface between InGaN and GaN, which is held to result in reduced sharpness of compositional change at the interface. A similar transition layer may also occur in the case of another material, for example, a gallium aluminum nitride (AlGaN) thin film heteroepitaxially grown on GaN crystals.
The above-described transition layer formed near the interface during heteroepitaxial growth having a nonuniform composition or having a lower composition may degrade characteristics of a semiconductor element that will be manufactured using the heteroepitaxial film. Therefore, performing heteroepitaxial growth such that a thin film sharply changes in composition at the interface while preventing a transition layer from occurring is important to improve the characteristics of a semiconductor element that will be manufactured using the heteroepitaxial film.
For example, Group III Nitride Semiconductor, Chapter 8 (Non-Patent Document 2) discloses a method of, when forming a multilayered structure of heteroepitaxial films, conducting a gas change valve having no dead space for instantaneously changing the supply of source gases so that a gas supply amount to a reaction tube and a gas supply amount to a vent are made substantially identical, in order to cause a sharp change in film composition at the interface.